This type of engine includes at least one cylinder comprising a combustion chamber delimited by the inner lateral wall of the cylinder, by the top of the piston that slides in this cylinder and by the cylinder head. Generally, a fuel mixture is contained in this combustion chamber and it undergoes a compression stage, then a combustion stage under the effect of a spark ignition, such as a spark plug, these stages being grouped together under the term “combustion stage” in the description hereafter.
It has been observed that this fuel mixture can undergo various combustion types and that these combustion types are the source of different pressure levels and of mechanical and/or thermal stresses some of which can seriously damage the engine.
The first combustion, referred to as conventional combustion or normal combustion, is the result of the propagation of the combustion of a fuel mixture compressed during a previous engine compression stage. This combustion normally propagates in a flame front from the spark generated at the plug and there is no risk it may damage the engine.
Another combustion type is a knocking combustion resulting from an unwanted self-ignition in the combustion chamber. Thus, after the fuel mixture compression stage, the plug is actuated so as to allow ignition of this fuel mixture. Under the effect of the pressure generated by the piston and of the heat released by the fuel mixture combustion start, a sudden and localized self-ignition of part of the compressed fuel mixture occurs before the flame front resulting from the ignition of the fuel mixture by the spark plug comes near. This mechanism, referred to as engine knock, leads to a local pressure increase and it can generate, in case it occurs repeatedly, destructive effects on the engine and mainly at the piston.
Finally, another combustion type is an abnormal combustion due to a pre-ignition of the fuel mixture before the spark plug initiates ignition of the fuel mixture present in the combustion chamber.
This abnormal combustion mainly affects engines that are the result of a downsizing operation. This operation is intended to reduce the size and/or the capacity of the engine while keeping the same power and/or the same torque as conventional engines. Generally, this type of engine is essentially of gasoline type and it is highly supercharged.
It has been observed that this abnormal combustion occurs at high loads and generally at low engine speeds, when timing of the fuel mixture combustion cannot be optimum because of engine knock. Considering the high pressures and the high temperatures reached in the combustion chamber as a result of supercharging, an abnormal combustion can start sporadically or continuously well before ignition of the fuel mixture by the spark plug takes place. This combustion is characterized by a first flame propagation phase that occurs too soon in relation to that of a conventional combustion. This propagation phase can be interrupted by a self-ignition involving a large part of the fuel mixture present in the combustion chamber.
If this abnormal combustion takes place repeatedly from engine cycle to engine cycle and starts from a hot spot of the cylinder for example, it is referred to as “pre-ignition”. If this combustion occurs suddenly, in a random and sporadic way, it is referred to as “rumble”.
The latter abnormal combustion leads to very high pressure levels (120 to 250 bars) and to a thermal transfer increase that may cause partial or total destruction of the moving elements of the engine, such as the piston or the piston rod.
The present invention therefore aims to identify the start of an abnormal combustion, of “pre-ignition” as well as of “rumble” type, with the devices and systems commonly used in engines. After identification, the invention is aimed to take the steps allowing to prevent this abnormal combustion from developing sufficiently in the engine cycle to have destructive consequences thereon.